1. Field of the Invention
The present invention relates to an apparatus for sensing NOx in a gas to be measured, having a sensor element made of an oxide, the resistance of the oxide varying in response to changes in NOx concentration of the gas if the oxide is contacted with the gas including NOx, and a measuring portion for measuring a resistance variation of the sensor element and for determining an NOx concentration in the gas to be measured. The present invention especially relates to an apparatus used preferably for sensing low concentration NOx in the atmosphere with respect to the NO concentration and NO.sub.2 concentration.
Moreover, the present invention relates to an apparatus for sensing NOx in a gas to be measured, having a sensor element made of an oxide, the resistance of the oxide varying in response to changes in NOx concentration of the gas if the oxide is contacted with the gas including NOx, and especially relates to an apparatus used preferably for sensing low concentration NOx in the atmosphere with respect to the NO concentration and NO.sub.2 concentration.
Further, the present invention relates to a chamber used for an apparatus for sensing a low concentration NOx having; a catalyst arranged to maintain partial pressures of NO and NO.sub.2 in the atmosphere at an equilibrium state; and sensor elements arranged in a flow path of the atmosphere, the resistance of each of the sensor elements varying in response to changes in NOx concentration of the atmosphere, one sensor element being contacted with the atmosphere which is not contacted with the catalyst and the other sensor element being contacted with the atmosphere which is contacted with the catalyst.
Furthermore, the present invention relates to a gas sensor element made of an oxide and a method of manufacturing the same, the resistance of the oxide varying in response to changes in NOx concentration of the gas to be measured if the oxide is contacted with the gas to be measured.
Moreover, the present invention relates to an ammonia removing apparatus for removing an ammonia component in a gas to be measured such as the atmosphere and an NOx sensor utilizing this ammonia removing apparatus.
2. Description of Related Art
As a method of measuring an NOx concentration in a gas to be measured such as a fired gas from an incinerator, which includes an NOx component such as nitrogen oxide, it is known to sample a gas to be measured, in, a dust chimney, and to measure an NOx concentration of the sampled gas by means of an optical measuring apparatus. However, the optical measuring apparatus is expensive and the response time thereof is long since the sampling operation is necessary.
In order to eliminate the drawbacks mentioned above, it has been proposed to use a direct insertion type semiconductor sensor. For example, in Japanese Patent Laid-Open Publication No. 6-222028 (JP-A-6-222028), an NOx sensor comprising a response portion made of an oxide having a predetermined perovskite structure, and a conductivity measuring portion for measuring a conductivity of the response portion is disclosed.
However, in the direct insertion type semiconductor sensor mentioned above, since a measuring target is an NOx concentration in a combusted exhaust gas flowing through the dust chimney, a temperature of the sensor element is low for example 300-400.degree. C. Therefore, NOx components is not adsorbed to a response portion in an equilibrium state.
This is not a problem if the apparatus is used for sensing an NOx concentration in a combusted exhaust gas having a relatively high NOx concentration. However, this presents a serious problem in an apparatus is used for sensing an NOx concentration in the atmosphere having a low NOx concentration, since it is not possible to measure an NOX concentration in with reliable precision.
Also, in the direct insertion type semiconductor sensor mentioned above, there is no countermeasure for an influence of O.sub.2 and CO components included in the gas to be measured with respect to the measured NOx concentration. Moreover, in the response portion, the resistance thereof is varied in response to the concentration of NOx(NO.sub.2 +NO). However, if a ratio of partial pressure between NO.sub.2 and NO, is varied, the resistance measured by the response portion will vary even for the same NOx amount. In this case, it is reasonable to conclude that the NOx component is not selectively measured. Therefore, in the direct insertion type semiconductor sensor mentioned above, there is a drawback in that the NOx concentration in the gas to be measured cannot be selectively measured in a highly precise manner, despite the fact that the semiconductor sensor is cheap and shows excellent response time as compared with the optical measuring apparatus.
Moreover, in order to solve the drawbacks mentioned above, the applicant discloses, in U.S. Pat. No. 5,705,129, an NOx sensor comprising; an oxide sensor element; a catalyst arranged upstream of the oxide sensor element to maintain partial pressures of NO and NO.sub.2 in the measurement gas at an equilibrium state; a heater for controlling a temperature of the oxide sensor element; and an O.sub.2 sensor for a correction. However, also in the NOx sensor mentioned above, a target measurement gas is a fired exhaust gas from an incinerator as mentioned above. Therefore, one drawback is that the NOx sensor mentioned above cannot be used for measuring low concentration NOx in the atmosphere which is a target measurement gas of the invention.
Further, the applicant discloses, in Japanese Patent Application No.9-80054, an apparatus for sensing a low concentration NOx having a catalyst arranged to maintain partial pressures of NO and NO.sub.2 in the atmosphere at an equilibrium state; and sensor elements arranged in a flow path of the atmosphere, the resistance of each of the sensor elements varying in response to changes in NOx concentration of the atmosphere, a first sensor element being contacted with the atmosphere which is not contacted with the catalyst and a second sensor element being contacted with the atmosphere which is contacted with the catalyst. In the apparatus for sensing a low concentration NOx mentioned above, it is disclosed that the first sensor element, the catalyst and the second sensor element are set in a chamber, but a construction of the chamber is not particularly defined at all. Therefore, it is required to make a construction of the chamber most suitable on making the chamber compact and on improving a measuring precision.
Furthermore, as a gas sensor element, there are known various types of gas sensor elements utilizing an oxide, which resistance is varied in response to changes in concentrations of components in a gas to be measured. As one example of these gas sensor element, there is disclosed in Japanese Patent Laid-Open Publication No. 61 155848 (JP-A-61-155848) a gas sensor utilizing an oxide in which a predetermined element is added in an oxide solid solution of titanium-tin system so as to make possible a measurement in Low temperature and to obtain compact and highly sensitive properties.
The above mentioned gas sensor element utilizing an oxide, in which precious metals are added, can realize the objects mentioned above. However, if it used for measuring an NOx concentration, an interference of SOx, HC and CO with respect to a measurement of NOx concentration becomes larger, and the influence of temperature variation in the case of measuring NOx concentration also becomes larger. Therefore, one drawback is measurement precision becomes worse.
Moreover, heated precious metals and oxides are used for the above mentioned catalyst. In this case, the catalyst functions not only to maintain partial pressures of NO and NO.sub.2 in a gas to be measured at an equilibrium state but also to generate an NOx component by oxidizing an ammonia component. As a result, if an ammonia component is existent in a gas to be measured, an NOx concentration in a gas to be measured which is passed through the catalyst becomes a sum of an NOx concentration existent in a gas to be measured from the beginning and an NOx concentration generated by oxidizing an ammonia component. Therefore, it is not possible to measure an NOx concentration of the gas in a highly precise manner.
On the other hand, as a method of removing an ammonia component which is not for an NOx sensor, there are known a method of using an adsorbent and a method of bubbling in an oxidizing solution. However, in the case of using the adsorbent, it is difficult to selectively adsorb an ammonia component and an NOx component is adsorbed together with an ammonia component. Therefore, it is not possible to measure an NOx concentration in a highly precise manner. Moreover, in the case of bubbling in an oxidizing solution, it is difficult to selectively dissolve an ammonia component, and a part of an NOx component is dissolved together with an ammonia component. Therefore, it is also not possible to measure an NOx concentration in a highly precise manner.